Determinants of Litter Accumulation and The

J For Res (2006) 11:313–318 © The Japanese Forest Society and Springer 2006 DOI 10.1007/s10310-006-0213-z

ORIGINAL ARTICLE

Shigenori Karasawa · Naoki Hijii Determinants of litter accumulation and the abundance of litter-associated microarthropods in bird’s nest ferns (Asplenium nidus complex) in the forest of Yambaru on Okinawa Island, southern Japan

Received: December 6, 2005 / Accepted: April 4, 2006

Abstract We studied the distributional pattern of bird’s Introduction nest ferns (Asplenium nidus complex) and the factors that determined litter accumulation and the abundance of litter- associated microarthropods in the ferns in the forest of Many researchers have reported that the presence of a Yambaru on the northern part of Okinawa Island, southern well-developed community of epiphytes can enhance the Japan. We located 53 bird’s nest ferns (41 ferns on 27 live nutrient-trapping capacity of forest systems in tropical and trees of 13 species, and 12 on 5 dead trees) in a ca. 4-ha plot, temperate regions, increasing their storage volume and act- collected litter samples from 37 ferns on 25 live trees, and ing as a “nutrient capacitor” (Benzing 2004) to steadily then extracted a total of 11205 microarthropods (Acari and release growth-limiting ions for other flora (e.g., Nadkarni Collembola) from all the litter samples. The ferns preferred 1984a, b; Nadkarni and Matelson 1992; Clark et al. 1998; concave slopes and tended to be distributed on the tree Nadkarni et al. 2002). species that had the typical characteristics of high popula- The bird’s nest fern (Asplenium nidus L. complex) is tion density and/or large basal area in the forest. The ferns widely distributed from tropical to subtropical regions (e.g., were usually established on large trees [≥10m high or Hsu et al. 2002; Ellwood and Foster 2004) and can grow to ≥20cm diameter at breast height (DBH)], although the a fresh weight of more than 200kg (Ellwood et al. 2002). number and size of the ferns were not related to the size of The basket-shaped rosette of long fronds can trap substan- the host trees. The amount of litter accumulated in the ferns tial amounts of leaf litter from the canopy, which contains was correlated neither with the size (height and DBH) of abundant and diverse litter-associated microarthropods the host tree nor with the height and position of the ferns. (Acari and Collembola) in tropical and subtropical forests The amount of accumulated litter had a significant positive (Rodgers and Kitching 1998; Walter et al. 1998; Karasawa correlation only with fern size; this might have caused the and Hijii 2006a, b). These litter-associated microarthropods positive correlations between fern size and the abundance have significant impacts on the decomposition processes of litter-associated microarthropods and the number of spe- in the forest floor and are important components of cies of oribatid mites in the ferns. biodiversity in various types of forest ecosystem (Wallwork 1983; Seastedt 1984; Lavelle and Spain 2001; Coleman et al. Key words Asplenium nidus · Bird’s nest fern · Litter · 2004; Karasawa and Hijii 2004a, b). Thus, clarification of the Microarthropod · Forest of Yambaru factors that determine litter accumulation in bird’s nest ferns and the community structures of the microarthropods associated with the litter could help us to understand the fern’s functions and effects on the diversity of invertebrate communities in forest ecosystems. In subtropical regions, however, few studies have examined how the spatial distribution and other traits of bird’s nest fern, as well as S. Karasawa1 (*) · N. Hijii their associations with host trees, can affect patterns of Laboratory of Forest Protection, Graduate School of Bioagricultural litter accumulation and the community structures of litter- Sciences, Nagoya University, Chikusa-ku, Nagoya, Japan associated microarthropods. Our aim was to reveal the factors that determine litter Present address: accumulation and the abundance of litter-associated 1 Iriomote Station, Tropical Biosphere Research Center, University of microarthropods in bird’s nest ferns in a subtropical forest the Ryukyus, 870 Uehara, Taketomi, Okinawa 907-1541, Japan Tel. +81-98-085-6560; Fax +81-98-085-6830 (the forest of Yambaru) on Okinawa Island, southern e-mail: [email protected] Japan. We first censused the distributional pattern of bird’s 314 nest ferns in the forest and then examined: (1) whether the size [height and diameter at breast height (DBH)] of host trees affected the size and number of bird’s nest ferns; (2) whether the size of the host tree and the location (height and position) and size of the ferns affected the amount of litter accumulated in the ferns; and (3) whether fern size affected the abundance of litter-associated microarthropods in the ferns.

Materials and methods

Study area

We conducted our study in an old-growth, evergreen, broad-leaved forest in the Yona Experimental Forest at the University of the Ryukyus, the forest of Yambaru, on the northern part of Okinawa Island in southern Japan (26°49′ N, 128°05′ E; 250–330m asl). The forest of Yambaru has an area of about 300km2, where more than 1000 plant species have been recorded (Itô 1997). In addition, the forest supports a large number of endemic and endangered insects, birds, and mammals (Itô et al. 2000), and is also characterized by high species diversities of insects and oribatid mites (Azuma et al. 1997; Itô et al. 1998; Itô and Aoki 1999). The area is characterized by a subtropical climate and Fig. 1. Projection areas of the rosette at the frond tips and of the litter accumulated in a bird’s nest fern (Asplenium nidus) abundant rainfall throughout the year. The mean annual temperature is 23.0°C, and annual precipitation between 1992 and 2003 averaged 2330mm (Experimental Forest at the University of the Ryukyus). The bedrock is composed From 18 October to 28 November 2003, we counted the of sandstone and slate, and the soil is classiﬁed as a yellow number of leaves on each fern; we also measured the pro- soil (Y) according to the Japanese Society of Forest Envi- jection areas of the rosette at the frond tips and of the ronment (1999). A ca. 4-ha plot containing ridges and val- region of each fern on which the litter was accumulated by leys has been established in the forest, which is dominated approximating both of these areas to an oval (Fig. 1). We by Castanopsis sieboldii (Makino) Hatus. with heights chose 37 ferns on 25 live trees of various species in the plot, ranging from 7 to 13m. In the past 50 years there has been removed all the litter from each fern, and placed the litter in no logging or other artiﬁcial disturbance in this forest separate plastic bags. (Shinzato et al. 1986; Enoki 2003).

Extraction of microarthropods Epiphytes We extracted invertebrates from all samples by using We conducted our measurements and sampling by ap- Tullgren funnels. Samples were first placed in the dark for proaching bird’s nest ferns using a single-rope technique 24h to avoid a sharp rise in temperature, which would have (Perry 1978; Whitacre 1981), so as to minimize the damage killed animals intolerant of desiccation, and then under 40- to the fern on the trunk of the host tree. All our fieldwork W electric bulbs for 72h. We sorted acari and collembolans was conducted between 0900 hours and 1700 hours in sunny from the invertebrates collected and counted them under a weather. Over four days in June 2003, we performed a binocular microscope (SZ6045TR; Olympus, Tokyo) at visual census of the spatial distribution of bird’s nest ferns in a magnification of 30×. Oribatid mites (Acari: Oribatida) the plot, identified their host-tree species, and then mea- were also sorted from acari, and then separated into adults sured the DBH and height of each host tree, as well as the and juveniles. Only the adults were identified (to height and position at which each fern was attached to each morphospecies level) and counted under a binocular micro- host tree. We divided these positions into six categories scope (BX41, Olympus) at a magnification of 400×. After based on Hsu et al. (2002): ground level; base of the trunk extraction of the invertebrates, we dried the litter samples up to 3m above ground level; the trunk; the main branching in an oven (70°C, 72h; DS-62; Yamato Scientific, Tokyo) point; the lower canopy (the primary branch); and the and weighed the dry litter on a microbalance (HF-200; upper canopy. A&D, Saitama, Japan). 315

Table 1. Number of bird’s nest ferns (Asplenium nidus complex) observed, the host trees to which the ferns were attached, and the dominance and distributional patterns of the host-tree species in this forest Tree species Number of Order of the dominance Distributional of host-tree speciesa,b patterna

Trees Ferns Density Basal area

Quercus miyagii 513154 B Distylium racemosum 463 3 A Castanopsis sieboldii 341 1 A Trupinia ternata 3323 28 C Schima wallichii 242 2 B Schefﬂera octophylla 234 5 C Diospyros morrisiana 2212 14 C Adinandra ryukyuensis 11<29 <29 – Pileostegia viburnoides 11<29 <29 – Meliosma squamulata 11<29 <29 – Machilus thunbergii 11<29 <29 – Meliosma rigida 11<29 <29 – Neolitsea sericea 11<29 <29 – Dead trees (standing) 3 5 Dead trees (fallen) 2 7 Total 32 53 B, Found in areas with an intermediate microtopography; A, found on gentle convex slopes; C, found on steep concave slopes; –, not investigated a Based on the data from Enoki (2003) b Smaller numbers represent higher ranks in dominance in the forest

Analyses Results We compared the mean number of leaves on the ferns Distributional pattern of bird’s nest ferns among three positions by the Kruskal–Wallis test using JMP software (SAS Institute 2002). We expressed the species Within the plot we located 53 bird’s nest ferns, which had 5 diversity of the oribatid community using Simpson’s diver- to 46 leaves each. All of the larger ferns were identified as sity index (1 − D), which is insensitive to sample size Asplenium nidus, but the small ferns could not be identified (Magurran 2004): to species level (presumably they were any of the three species Asplenium antiquum Makino, A. nidus, or Asple- DnnNN=−∑[]()11[]()− ii nium setoi Murak. et Seriz; Murakami et al. 1999). We found 41 ferns on 27 live trees of 13 species, and 12 on 5 where ni denotes the number of individuals in the ith species and N denotes the total number of individuals. dead trees (tree species not identified) (Table 1); however, We analyzed four categories of correlations by using because 4 ferns on 2 live trees had died by 18 October, we Pearson’s correlation coefficient (r) or analysis of variance used only live ferns on live trees (37 ferns on 25 trees) for (ANOVA): (1) between the size (height and DBH) of each the extraction of microarthropods. Among the tree species host tree and the number of bird’s nest ferns on the tree, the with more than 2 ferns, Castanopsis sieboldii, Schima number of leaves on each fern, (2) between the number of wallichii Korth, Distylium racemosum Sieb. et Zucc., and leaves on each fern and the two projection areas on the fern Schefflera octophylla (Lour.) Harms were dominant in (as described above); (3) between the size of the host tree, terms of density. The basal area of Quercus miyagii the location (height and position) of the fern, the number of Koizumi was larger than that of other species in the forest, leaves on the fern, and the amount of litter accumulated in although the tree density was low. Diospyros morrisiana each fern; (4) between the number of leaves on each fern Hance and Trupinia ternata Nakai, both of which were and the abundance of all litter-associated microarthropods, found on steep concave slopes, were present at lower densi- the number of species of oribatid mites, their species diver- ties than other species in this forest (Enoki 2003; Table 1). sity in the fern. All variables except the position of the ferns Bird’s nest ferns were attached only to the trunk bases were analyzed after log-transformation. (15 ferns), trunks (23 ferns), and main branching points (15 ferns) of the host trees, and there was no significant differ- ence in the number of leaves on these ferns as a function of the fern’s position (P > 0.05). The ferns were found at heights of 0.1–10.5m above ground level, and there was no close relationship between the numbers of ferns and leaves and the heights above ground level of the ferns. 316 The height and DBH of the live host trees ranged from 7.0 Litter accumulation and the abundance of to 15.5m and from 4.8 to 46.2cm, respectively. More than microarthropods in bird’s nest ferns 85% of all the ferns that were attached to the live trees were found on the trunks of trees higher than 10m, and the largest The amount of litter accumulated in the ferns reached ca. number of ferns per tree (7 ferns) was found on the highest 120g dry weight, and it was correlated neither with the size tree (15.5m). The height of the host tree, however, was not (height and DBH) of the host tree nor with the height and significantly correlated with the number of ferns per tree (r = position of the fern (Fig. 2A–D; P > 0.05). There was a 0.350; P > 0.05). The largest number of ferns per tree was not positive correlation only between the amount of litter and found on the host tree with the largest DBH, and there was the number of leaves on the fern (Fig. 2E; r = 0.883, no significant correlation between DBH and the number of P < 0.0001). ferns per tree (r = 0.205; P > 0.05), although more than 75% of We collected 11205 microarthropods from all the the ferns censused occurred on the trunks of the trees with a litter samples: 7755 acari including oribatid mites (3671 DBH larger than 20cm. As in the case of the number of ferns individuals in 79 species) and 3450 collembolans. We found per tree, the number of leaves on the fern was not correlated significant correlations when we compared the number of with the height (r = −0.137, P > 0.05) and DBH (r = −0.095, fern leaves with the numbers of individuals of acari and P > 0.05) of the host tree. collembolans, and the numbers of individuals and species of The projection area of the rosette at the frond tips oribatid mites. There was no correlation, however, between reached up to 3.3m2, and the projection area of the region the number of fern leaves and the species diversity of orib- on which the litter was accumulated in the fern ranged from atid mites (Table 2). Details of the oribatid mites in the 13.0 to 551.1cm2. Both areas were significantly correlated ferns are described in Karasawa and Hijii (2006a). These with the number of leaves on the fern (rosette: r = 0.921, close relationships between various variables derived from P < 0.0001; litter: r = 0.883, P < 0.0001). the bird’s nest ferns and the number of leaves on each fern

Fig. 2. Relationships between the amount of the litter accumulated in bird’s nest ferns (Asplenium nidus complex), and the height (A) and diameter at breast height (DBH) (B) of the host tree, the height (C) and position (D) of bird’s nest ferns, and the number of leaves on the ferns (E). Based on the data of 37 ferns attached to the live trees. Pearson’s correlation coefficient was used for A, B, C, E, and analysis of variance (ANOVA) was used for D 317 Table 2. Correlations of the numbers of individuals of Acari and favorable conditions such as high humidity on concave Collembola, and the numbers of individuals and species and species slopes, bird’s nest ferns may have become established on diversity (1 − D) of Oribatida, with the size (no. of leaves) of bird’s nest ferns (Asplenium nidus complex) the tree species that were dominant in the area and only on those trees that were large enough to support the weight of Taxon nrthe ferns. Acari 37 0.769* In this forest, we found no ferns on the ground or in the Collembola 37 0.713* canopy, even in large trees. Hsu et al. (2002) reported that Oribatida bird’s nest ferns grew on the trunk base, main trunk, and No. of individuals 37 0.758* No. of species 37 0.724* main branching point of the host trees in a moist subtropical Species diversity 31a 0.109 forest in northeastern Taiwan. These findings suggest that the vertical distribution of the ferns on host trees is limited *Pearson’s correlation coefficient, P < 0.0001 a Six samples with no or one adult were excluded by the structure of the trees. This distributional pattern of ferns may be characteristic of subtropical forests in eastern Asia (Hsu et al. 2002).

Table 3. Regressions of variables derived from the bird’s nest ferns (Asplenium nidus complex) on the number of leaves on each fern Factors limiting litter accumulation and the abundance of 2 Equations nr P microarthropods in bird’s nest ferns = − < LogW 2.36LogNL 1.71 37 0.786 0.0001 = − < LogNA 2.36 LogNL 1.06 37 0.591 0.0001 The amount of litter accumulated in the bird’s nest ferns = − < LogNC 2.18LogNL 1.20 37 0.509 0.0001 had a clear positive correlation only with fern size; this LogN = 2.24LogN − 1.27 37 0.575 <0.0001 O L might have caused the positive correlations between fern LogN = 1.03LogN − 0.44 37 0.524 <0.0001 OS L size and the abundance of litter-associated microarthropods W, Dry weight of the litter accumulated in each fern; NL, number of and the number of species of oribatid mites in the ferns. On leaves on each fern; N , number of individuals of acari (including A the other hand, no significant correlation of the oribatid oribatid mites); N , number of individuals of collembolans; N , number C O species diversity with fern size may be caused by irrelevance of individuals of oribatid mites; NOS, number of species of oribatid mites of habitat heterogeneity and the number of trophic niches to fern size (Karasawa and Hijii 2006a). Some previous studies have reported that the size of epiphytes influences were also quantified by linear regressions on log–log the number of species and the biomass of invertebrate coordinates (Table 3). communities living in them (Richardson 1999; Ellwood and Foster 2004; Karasawa and Hijii 2006a), as well as the guild composition and ecosystem function (Stuntz et al. 2002; Discussion Wardle et al. 2003). Moreover, Martin et al. (2004) reported that the concentrations of carbon, calcium, and potassium

Habitat preference of bird’s nest ferns and rates of net CO2 exchange in the leaves of A. nidus in Taiwan varied with fern size, although most other physi- The significant positive correlations between the number of ological parameters did not vary with fern size. Thus, varia- leaves and the projection areas of both the rosette formed tions in the size of the bird’s nest ferns can maintain the by the frond tips and the region on which the litter was complexity of nutrient cycling and may affect the structure accumulated in each fern indicated that the number of of microarthropod communities in the canopy. leaves could be used as an indicator of the size of the fern, Moreover, close relationships between the number of which has a basket-shaped rosette. leaves on each fern and variables derived from the ferns About 85% of the bird’s nest ferns on live trees (i.e., (Table 3) enable us to obtain approximate estimates of the identified to species) occurred on host-tree species that dry weight of the litter accumulated, and the numbers of had the typical characteristics of higher density and larger individuals of litter-associated microarthropods and species basal area (four species in a higher rank in all the live trees) of oribatid mites in each fern in the forest of Yambaru, or location on a concave slope. The number and size of simply by counting the number of leaves on a fern. A similar the ferns did not increase with increasing host-tree size, relationship was established between fern size and all although many ferns were found on larger trees more invertebrate biomass in each fern in a tropical rainforest than 10m high and/or with greater than 20cm DBH. (Ellwood and Foster 2004). Annaselvam and Parthasarathy (2001) reported that Asple- In conclusion, the bird’s nest ferns preferred concave nium nidus occurred on a large number of host species (62 slopes and were usually established on large trees (≥10m species) and on trunks more than ca. 20cm DBH in a tropi- high or ≥20cm DBH) of the dominant species in this sub- cal evergreen forest in India. Shaw (2004) pointed out that tropical forest. Litter accumulation in each fern was deter- topographic and environmental factors within forests, such mined only by the size of the basket-shaped rosette of the as slope position, proximity to valley bottoms, and overall fern, and this factor could affect the abundance of litter- moisture availability, have a complex influence on the associated microarthropods and the number of species of position of epiphytes on trees. Thus, it is likely that under oribatid mites in the ferns. On the basis of these specific 318 relationships, we presented procedures for conveniently es- Karasawa S, Hijii N (2004b) Morphological modifications among orib- timating the amount of litter, and the numbers of individu- atid mites (Acari: Oribatida) in relation to habitat differentiation in mangrove forests. 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